email/worker: add more context to errors

For instance, failure to open the private PGP key would just fail
with an unhelpful "panic: open : no such file or directory" error.

email.EnqueueStd: Don't overwrite headers

Sometimes we need to specify the Message-Id, From, and Reply-To headers
(e.g. for todo.sr.ht ticket notifications). Don't overwrite these
headers if they are present.

email: handle failure to encrypt notifications

Currently, failure to encrypt the notification email - e.g. due to an
expired key - will make the entire request fail. Instead, fall back to
sending an unencrypted email and let the request succeed (unless that
fails also, which is unlikely).

go fmt

Replace x/crypto package by ProtonMail/go-crypto to support ed25519 OpenPGP keys

go fmt

Remove %e formatting verbs

%e is not valid for formatting error values:

> %e	scientific notation, e.g. -1.234456e+78

Instead, use %w when using fmt.Errorf (for error wrapping) and
%v when using log.Printf (%w is not valid in that context).

Configure server to manage mail queue

This also sets up the recovery function for webhooks, so we get emailed
when we panic during a webhook query.

Encrypt outgoing emails

Sign outgoing emails

Make worker logs less loud

Expand context for work queues

Add email work queue